1. Field of the Invention
The present invention relates to a total internal reflection-type optical waveguide switch, and more particularly, to a total internal reflection-type optical waveguide switch lowered in the level of initial crosstalk at the time of switching operation.
2. Description of the Prior Art
In the field of optical communication, a total internal reflection-type optical switch is used for optical path switching. A semiconductor-type optical waveguide switch, as an example of this total internal reflection-type optical switch, will now be described with reference to the accompanying drawings.
FIG. 1 is a schematic plan view showing a conventional example of the total internal reflection-type optical waveguide switch. In FIG. 1, two optical waveguides 1 and 2 cross at an angle .theta., thus forming a crossing section 3. If light is incident as indicated by arrow p, optical waveguides 1a and 2a both function as incidence-side optical waveguides, while optical waveguides 1b and 2b both function as emission-side optical waveguides. A refractive index changing region 4 is formed at the crossing section 3 between the optical waveguides.
The portion of each optical waveguide other than the refractive index changing region 4 is constructed in the manner shown in the sectional view of FIG. 2, which is taken along line II--II of FIG. 1. If GaAs and AlGaAs are used as semiconductor materials, for example, a substrate 12 of GaAs is formed on the back of a lower electrode 11, and n.sup.+ GaAs is built up to form a buffer layer 13 on the substrate 12.
A lower cladding layer 14 of n.sup.+ AlGaAs and a core layer 15 of n.sup.- GaAs are successively formed on the buffer layer 13. Then, an upper cladding layer 16 of n.sup.- AlGaAs and a cap layer 17 of n.sup.- GaAs are successively formed on the core layer 15, and are etched to form a ridge-shaped optical waveguide. The whole resulting structure is covered by an insulating film 18 of SiO.sub.2 or the like.
As shown in the sectional view of FIG. 3, which is taken along line III--III of FIG. 1, on the other hand, a diffused region 4a is formed in the refractive index changing region 4 by diffusing a predetermined amount of an impurity, such as Zn, into the upper cladding layer 16 so that the impurity nearly reaches the core layer 15. A slit-shaped insulating film aperture 18a is formed by partially removing the insulating film 18, which covers the surface of the crossing section 3, for a predetermined width with respect to the longitudinal direction of the crossing section 3. Then, an upper electrode 19 is mounted on the insulating film aperture 18a.
In the case of this optical waveguide switch, if light is applied to the incidence-side optical waveguide 2a, as indicated by arrow p in FIG. 1, without causing any operation between the upper and lower electrodes 19 and 11, the light directly advances straight through the crossing section 3, and emerges from the emission-side optical waveguide 2b, as indicated by arrow q.
If electric current of a predetermined value, for example, is injected from the upper electrode 19 via the insulating film aperture 18a, however, the refractive index of that portion of the core layer 15 which is situated under the refractive index changing region 4 is lowered. As a result, a boundary surface 4b between the refractive index changing region 4, whose refractive index is lowered, and the optical waveguides, whose refractive index is not lowered, develops at the crossing section 3. The boundary surface 4b is situated on a straight line which bisects the intersection angle .theta..
Accordingly, the incident light upon the incidence-side optical waveguide 2a changes its optical path toward the emission-side optical waveguide 1b, with the boundary surface 4b used as a reflective surface, as indicated by broken line q' in FIG. 1, and then emerges from the waveguide 1b. Thus, the boundary surface 4b of the refractive index changing region 4 is made to be a totally reflective surface by the current injection, and the light is totally reflected by this surface, so that a switching function develops.
In the case of the optical waveguide switch described above, however, the insulating film aperture has the form of a slit, so that the boundary surface 4b of the refractive index changing region 4, which is formed by diffusing Zn through the insulating film aperture, has a flat configuration.
At the crossing section 3, the refractive index changing region 4 contains different materials, so that the refractive index of the remaining portions of the optical waveguides is finely different from that of the region 4. Even when no electric current is injected from the electrode 19, therefore, the light incident upon the incidence-side optical waveguide 2a is reflected or refracted by the boundary surface 4b, resulting in an optical loss. Thus, the initial crosstalk level in the through state is high.